1. Field of the Invention:
This invention relates to an improvement of a process for synthesizing carboxylic acids by the carbonylation of alcohol. More particularity, this invention relates to the addition of trihaloacetic acid to the rhodium catalytic system to promote the reaction rate. The alcohol used in this reaction system has the formula of ROH, where R is a saturated hydrocarbonyl radical having 1 to 4 carbon atoms.
2. Description of the Prior Art:
The use of group VIII transition metals as the carbonylation catalysts for the production of carbonylation products are known in the art, with many of such catalysts being based on cobalt and using a halide promoter. However, the rhodium system has become the most important one in the recent years. So far as we know, there has not heretofore been a disclosure of a rhodium based carbonylation catalyst system containing trihaloacetic acid as the promoter used in the generation of carboxylic acid such as acetic acid by the carbonylation of alcohol such as methanol with carbon monoxide at mild pressure and temperature conditions. The rhodium catalytic systems that have been disclosed will be described in the following paragraphs and the relationship between those procedures and this invention will be discussed.
Among processes for synthesizing acetic acid, the one that is the most commercially useful is the carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 and in U.S. Pat. No. 4,690,912 both issued to Paulik et al. on Oct. 30, 1973, and Sep. 1, 1987 respectively. The reaction was carried out at 180.degree. C. and 35-70 Kg/cm.sup.2 Co. The catalyst system comprises rhodium, either dissolved or otherwise dispersed in the liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter such as methyl iodide. The patents teach that the preferred solvent and liquid reaction medium for the process is the desired carboxylic acid itself, i.e., acetic acid when methanol is being carbonylated to produce acetic acid. The selectivity is exceptionally high, typically greater than 95%. However, the rate of carbonylation has been highly dependent on water concentration in the reaction medium. When the water concentration is reduced to below about 14-15 wt %, the rate of reaction declines. Reducing the water content also leads to the formation of by-product such as ester. It is taught in European Patent No. 0055618 that typically about 14-15% water is present in the reaction medium of a typical acetic acid-producing system using carbonylation technology. In a paper written by Hjortkjaer and Jensen [Ind. Eng. Chem., Prod. Res. Rev. 16,281 (1977)], it is shown that the reaction rate of methanol carbonylation is increased with the increasing of water content from 0 to 14 wt %. Above 14 wt %, the reaction rate is unchanged. The catalyst also becomes more susceptible to inactivation and precipitation when carbon monoxide partial pressure and/or water content is/are low. These prior arts disclose an important message that water must be present in the reaction system to obtain a satisfactory reaction rate.
Taiwan Application No. 74101387, to Celanese Corporation, disclose that using high concentration of LiI as the stabilizer for the rhodium catalytic system under a limited amount of water, it will be necessary to add methyl acetate as the promoter in order to obtain a productivity almost the same as the one which was obtained from a system with high concentration of water. The purpose of the Celanese patent is to reduce the amount of water but this also reduces the reaction rate.
It can be seen from the prior arts mentioned above that those systems either have the disadvantage of requiring high water content or have the disadvantage of the reduced reaction rate if the amount of water therein is reduced. Those literatures have never mentioned the use of trihaloacetic acid. Therefore, the main purpose of this invention is to find a suitable additive which can both reduce the amount of water and increase the reaction rate. Using the catalytic system of this invention, a very high productivity can be obtained under the reduce water content of 4 to 11% so that the problem resulting from water can be avoided. The catalytic system of this invention, even though dissolved in the system containing low amount of water, is still resistant to precipitation. Furthermore, in view of the separation of the acetic acid from low amount of water, the consumption of energy during distillation can be reduced when compared with the separation of them from high amount of water. The other procedures, such as solvent extraction, can also be simplified and thus the expansion of equipments can be avoided. The accomplishment of this invention, i.e., achieving high productivity and reaction rate under low water content by the incorporation of trifluoroacetic acid into the catalytic system, can not be overmatched by the prior art. According to the present invention, the reaction rate in the environment of low water content and in the absence of methyl acetate is even higher than that of the prior art in which methyl acetate is used.